1. Field of the Invention
The present invention relates to acicular ferromagnetic iron oxide particles, and more particularly acicular ferromagnetic iron oxide particles suitable for magnetically recording mediums capable of recording sounds and images in a high density, a powdery acicular magnetic iron oxide useful as starting material for production of such particles, and a process for producing the same.
2. Description of Related Art
Recently, there has been an increasing need to enhance the performance of magnetically recording mediums such as magnetic tapes, magnetic disks, magnetic drums and the like, as apparatuses for magnetically recording and playing back sounds and images have been miniaturized and upgraded and as a magnetic information processing capacity has been greatly increased. That is, further enhancements in properties such as a high recording density, a high output and the like, have been increasingly demanded, which requires in turn specifically magnetic particles used in the magnetically recording mediums to be much finer and to have a higher coercive force and a higher saturation magnetization characteristic.
Although making magnetic particles finer is one of the most effective methods for reducing noises, it causes inevitably a reduction in the saturation magnetization of the particles as well as reductions in a filling ratio and an orientability of the magnetic particles in a magnetic layer containing them dispersed as the sizes of the particles are reduced. Therefore, there is a great need to further enhance the magnetization properties of the magnetic particles such as saturation magnetization and remanent magnetization so that the magnetically recording mediums can meet the requirements of having a higher recording density and a higher output. Thus, various techniques have been proposed to improve the magnetic properties of the magnetic particles for use in the magnetically recording mediums. For example, there have been known a method for improving the print-through property as well as the saturation magnetization by heat-treating an iron oxide powder to densify the crystalline texture (for example, Japanese Patent KOKAI (Laid-Open) No. Sho 58-199725), or a method for improving the print-through property as well as the saturation magnetization by modifying maghemite particles through coating with a combination of ferrous compounds and zinc compounds and, if necessary, by further heat-treating the coated particles to avoid reductions in the print-through property and the saturation magnetization due to the coating-treatment when the maghemite particles are modified through coating with cobalt compounds for increasing the coercive force (for example, Japanese Patent KOKAI (Laid-Open) Nos. Sho 53-87961, 60-208805, and 61-4202). In the former case, however, an attempt to obtain a sufficient print-through property tends to form .alpha.-Fe.sub.2 O.sub.3, to cause unavoidably deterioration in the saturation magnetization, and in the latter case, the coercive force varies to an large extent with time, though the coercive force and the saturation magnetization may be improved to a certain degree. Therefore, there have not been a few of difficulties to be improved yet.
Moreover, in order to provide magnetic particles which can meet the demands on the market as described above, the have been proposed those comprising primarily iron and those of berthollide particles having a crystalline structure partly modified with zinc ions which are produced by heat-treating berthollide particles having a zinc component, or heat-treating magnetite particles having a zinc component followed by oxidation treatment.
The magnetic particles comprising primarily iron as discussed above have characteristics of a high coercive force and a high saturation magnetization, but disadvantageously are expensive and not easily handled because of being highly prone to oxidation. Therefore, there have been a need to provide iron oxide based magnetic particles which are inexpensive and easy to handle and have higher coercive force and higher saturation magnetization properties.
The present inventors have been making various researches on improvement of the performance of magnetic iron oxide particles for the purpose of providing those which are capable of meeting the demand for higher recording densities and higher outputs and lower noise levels to be achieved by the magnetically recording mediums and have made an advancement in an attempt to improve the saturation magnetization of the particles through modification with a minor amount of metallic ions. In U.S. Pat. No. 5,041,307, it was proposed that by heat-treating berthollide particles containing a specific amount of zinc ions under specific heating conditions, or by heat-treating magnetite particles containing a specific amount of zinc ions under specific heating conditions and then subjecting the heat-treated particles to an oxidation-treatment allowing the ferrous ions to be included, the crystalline particles are modified with the zinc ions and the ferrous ions, whereby the synergetic effects of the zinc ions and the ferrous ions allow production of berthollide (FeOx, 1.33&lt;x&lt;1.5: that is, an intermediate oxide between magnetite and maghemite) particles having a much higher saturation magnetization, and in addition stable magnetic properties, a high dispersibility to coating composition and an excellent squareness. Moreover, it has been found that such berthollide particles may be coated with a cobalt compound or both a cobalt compound and a ferrous compound to achieve much more desirable magnetic properties so that they become extremely suitable to high density magnetic tapes for recording sounds and images.
In the cource of the investigation to improve the magnetic properties, the present inventors directed attention to and studied the modification of berthollide compounds, an intermediate oxide between magnetite and maghemite, with zinc ions. As a result, it has been found that specific berthollide particles having a higher performance can be produced.